Porous barrier for evenly distributed purge gas in a microenvironment

ABSTRACT

An improved system and method for purging a microenvironment to desired levels of relative humidity, oxygen, or particulates through the implementation of a purge gas delivery apparatus and method that provides even distribution of the purging gas within the microenvironment. A substrate container has a tower therein with a fluid flow passageway extending the length of the tower. Apertures with porous media between the aperture and fluid flow passageway regulate the volume and pressure of air discharging at each aperture. Alternatively, the tower may be formed of a porous tubular polymeric material. A sleeve may direct the discharge purge gas in the interior.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/285,218, filed Dec. 10, 2009, which is incorporatedherein in its entirety by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to transportable substrate containerssuitable for use in storing or transporting objects such assemiconductor wafers in an extremely clean environment and moreparticularly to systems and methods for purging the environment withinsaid container to desired levels of relative humidity, oxygen, andairborne particulates.

BACKGROUND OF THE INVENTION

During transport or storage of substrates, if traces of dust or gaseousimpurities present in the surrounding air adhere to the semiconductorwafers or other objects, the subsequent product yield from the affectedwafer is lowered. This tendency becomes increasingly noticeable as thedegree of integration increases. Accordingly, there is an increaseddemand for the environment within these transport containers to achievea high level of cleanliness regarding not only dust but also gaseousimpurities.

Currently to produce a clean space for accommodating substrates whentransported or stored, microenvironments are purged using an inert gaswhich is injected into the interior of the container through an inletport causing the air within the container to exit through an outletport. These purge gas inlet and outlet ports are generally located onthe bottom surface of the container shell and extend from the interiorthrough the shell bottom surface where they interface with the purge gasdelivery system. This method uses a check valve and circular PTFEparticulate filter located in the purge inlet port to control and filterthe incoming purge gas. The inert purge gas then replaces the air andcontaminants within the container and the contaminated air is forced outof the container through the purge outlet port or the door opening ifthe door is removed.

Other methods of delivering purge gas into the container have beendesigned to provide the improved environment within the container, theseprior art attempts have significant drawbacks which precipitate the needfor the present invention. For example, U.S. Pat. No. 6,221,163 toRoberson et al. discloses a system and method for molecularcontamination control permitting purging a container to desired levelsof relative humidity, oxygen, or particulates. The container includes aninlet port and an outlet port, each including a check valve and filterassembly for supplying a clean, dry gaseous working fluid to maintainlow levels of moisture, oxygen, or particulates. The inlet port isconnected with a gaseous working fluid source and the outlet port isconnected to an evacuation system. The integral directional flow checkvalves operate at very low pressure differentials (such as less than 10millibar). In one embodiment, flow of purge gas inside the container canbe directed towards the substrates with one or more nozzle towers toencourage laminar flow inside the pod. One or more outlet towers, havinga similar function to that of the inlet tower may also be provided toencourage laminar flow inside the container. This method has aparticulate filter located in the check valve/filter assembly located inthe bottom of the tower. The disadvantage of this method is that as thepurge gas flows through the orifices of the tower, the distribution ofgas decreases from the tower top orifice to the tower bottom orifice.This results in an extended time to evacuate the water and oxygen fromthe microenvironment.

One goal of purging is to decrease humidity and oxygen levels in theinternal volume of the microenvironment. A challenge of purging is toevenly distribute the purge gas fast and effectively over the substrateswithin the container. Current purging methods, using inlet and outletports at the bottom of the container do not meet either requirement ofpurging fast or effectively because the path of least resistance for thepurge gas flow is directly from the inlet to the outlet. Little of thegas flows between the wafers, and the volume between the wafers is adead zone that is not immediately affected by the purge gas. Thisresults in non-uniform humidity and oxygen levels within themicroenvironment. Utilizing the purge tower improves the speed andeffectiveness of the purge, but the orifices in the tower still do notevenly distribute the purge gas between the wafers even with theimplementation of graduated orifices. In effect, as the purge gas flowsthrough the orifices of the tower, the distribution of gas decreasesfrom the top orifice to the bottom orifice. This results in ineffectiveevacuation of water and oxygen from the microenvironment.

Another goal of purging is to remove airborne contaminants from theenvironment within the container. These contaminants are primarilytrapped in the filtering mechanism in the purge ports and areeffectively removed when the container is purged. However, thecontainers are reusable and prior to reuse must be cleaned. As a part ofthe cleaning process, a human operator must remove the purge tower andthe purge port/filter mechanisms to keep water from accumulating inthese devices. Contaminants are introduced into the microenvironmentfrom the human operator during the process of removing/reinstalling thepurge towers and purge ports/filters.

A need therefore exists for an improved purging method which solves theproblems of evacuating the wafer container in a fast and efficientmanner. In particular, a need exists for a purging method and apparatusthat can allow the purge gas to accumulate in the tower so that as thegas accumulates and the pressure inside of the tower increases thereforethe gas flows through the tower into the microenvironment and is evenlydistributed between the wafers. Another need is to minimize or eliminatehuman operators from reaching inside the container. Currently, operatorsmust reach inside the container to remove and reinstall the purge towersand purge ports/filters when the container is cleaned and returned toservice.

SUMMARY OF THE INVENTION

In view of the above mentioned problems, the present invention providesfor an improved system and method for purging a microenvironment todesired levels of relative humidity, oxygen, or particulates through theimplementation of a purge gas delivery apparatus and method thatprovides even distribution of the purging gas within themicroenvironment.

An additional advantage of the present invention is that the purge towerporous barrier may be composed of a hydrophobic material that preventswater from entering the purge tower during the cleaning process whichconsequently eliminates the need to remove the purge gas deliveryapparatus during the cleaning process which avoids introducing particlesinto the microenvironment from the removal and reinstallation process ofthe purge gas delivery apparatus after cleaning.

Further advantages of the invention are that the porous barrier may beformed into shapes that match the non-flat geometry of the purge towerand by resisting the flow of gas, the porous barrier allows for thepurge gas to accumulate in the tower. As the gas accumulates, thepressure inside the tower increases and when the pressure is sufficientto overcome the barrier material, the gas flows through the barriermaterial into the wafer container and is evenly distributed among theinstalled wafers in the container.

A further advantage of the invention is that the purge towersignificantly reduces the relative humidity level within themicroenvironment in instances where the wafer access door has beenremoved.

A further advantage of the invention is that the purge tower porousbarrier improved purging efficiency from top to bottom of themicroenvironment.

In one preferred embodiment, the microenvironment includes an inlet portand an outlet port, each including a check valve assembly and a purgetower connected to the inlet port and the outlet port for a clean, drygaseous working fluid that is used to provide controlled low levels ofmoisture, oxygen, and particulate content around the materials containedin the microenvironment. The microenvironment container inlet port isconnected with a gaseous working fluid source, and the outlet port isconnected with an evacuation system. In operation, according to oneembodiment, the tower includes a sealing grommet in the tower baseportion that maintains a seal against the container shell to preventundesired chemicals or particulates from entering into the interior ofthe wafer container. Thus, any flow between the interior and exterior ofthe wafer container is limited to passing through the passageway definedby the grommet. Types of fluid flows include the introduction of purginggases such as nitrogen into the interior of the wafer container. Thefluid flow can be further limited by the porous barrier element. Theporous barrier provides filtering to the purge gas to removeparticulates as well as oxygen and humidity from the purge gas prior tothe gas being introduced into the interior of the wafer container. Theintegral directional flow check valves operate at very low pressuredifferentials. In this embodiment, a porous membrane barrier is placedin the inlet tower directly behind the tower orifices. The membranebarrier improves the performance of the tower in two ways. For the firstimprovement, the membrane barrier allows for the purge gas to evenlypressurize within the tower. The pressurized gas then evenly flowsthrough the membrane and the tower orifices and into the containerinterior. The orifices in the tower can be of any shape including acontinuous vertical opening from top to bottom of the tower such thatthe tower has a “C” shape as viewed from the top down. This results inevenly distributed purge gas between the substrates in themicroenvironment and the tower resulting in an improved reduction inrelative humidity and oxygen from the microenvironment. For the secondimprovement, the barrier can be manufactured from a material withhydrophobic characteristics that act as a water barrier during thecleaning of the microenvironment. This hydrophobic water barrier keepswater out of the purge tower and consequently the purge port during thecleaning process. The porous membrane barrier also functions as aparticle filter which removes particles from the gas flow whicheliminates the need for a particle filter in the purge port at the baseof the tower.

In an additional embodiment, the membrane barrier is manufactured out ofa porous polymer that is formable into geometry that matches thegeometry of the tower. A benefit of the formed barrier is that it canalso be used to seal the mating surfaces of the rear and slotted frontportions that make up the purge tower. Additionally, the barrier can beformed into a flat woven shape and used to cover the orifices in thefront portion of the tower.

An additional embodiment, the purge tower itself can be fabricated fromthe same porous polymer material as is used for the membrane barrieravoiding the need for orifices, radial slots, or separate tower portionsto form the tower. This tower may be manufactured as a capped poroustube that attaches directly to the inlet/outlet ports on the bottomsurface of the container. This tower can be made as a “C” shaped sleevethat can be situated such that flow is directed towards the substratesor towards the container wall, or any other direction desired simply byhow the sleeve is positioned in the microenvironment.

An additional embodiment is that the porous membrane filter can be madeas a bladder that is installed within the tower. In this embodiment, theporous material not only covers the purge tower nozzles but also thereveal between the front and back portions of the purge tower. Thisallows for any leakage through the reveal to be filtered as well as anywater to be blocked from coming into the tower through the reveal. Thepurge gas is injected into the filter bladder at the base of the towerand subsequently is dispersed through the bladder walls, through thetower slots and into the container interior. The bladder can bemanufactured either as a complete tube that is installed between thefront and back portions of the tower, or as a complete tube that isinstalled between the front and back portions of the tower.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a front opening wafer container.

FIG. 1B shows a front opening wafer container with one purge towerinstalled.

FIG. 2 is a front view of the tower showing the gas dispersal slots

FIG. 3. is an exploded view of the back portion of the tower, the porousmembrane barrier and the slotted front portion of the tower

FIG. 4 shows the purge gas flow into the tower and subsequently outthrough the barrier membrane and the tower slots into the containerinterior.

FIG. 5 is an exploded view of the purge tower made of porous polymermaterial which is then press-fit into the molded base portion.

FIG. 6 is an exploded view of the tower and porous membrane bladder.

FIG. 7 is a cross section view of the complete tube shape bladdershowing the gas inlets and the different wall section thickness whichenables the purge gas flow toward the disperse slots in the frontportion of the tower.

FIG. 8 shows the respective flow data for porous polyethylene and porouspolypropylene in the presence of air pressure and water pressure.

FIG. 9 shows the reduction in relative humidity measured at wafer slot#1, wafer slot #13 and wafer slot #25 with the door removed before andafter purge with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A illustrates a configuration of a substrate container configuredas a wafer container assembly 1 known as a FOUP or FOSB comprising anopen front 2, a front door 3, and an enclosure portion 4. Wafers W areinserted and removed horizontally through the open front. Slots formedin the interior sides hold the substrates, in this case semiconductorwafers. Front door 3 with seal sealingly engages with enclosure portions4 to form an interior space that is isolated from the ambientatmosphere. Receiving structure 5 for the purge towers 6 may be in thebottom wall of the enclosure portion.

Referring to FIG. 1B, a purge tower 6 is shown that is part of thepreferred embodiment enabling gaseous working fluid or purge gas insidea substrate container to be directed toward and away from substrateshoused in the container using one or more uniquely configured towers 6connected by a base portion 7 to the inlet port or the outlet port.Orifices are provided in each tower, preferable in the form of a seriesof spaced apart nozzles 8. The gaseous purge fluid will sweep thecontainer and its contents picking up residual moisture, oxygen andAMC's and encouraging movement of particulates toward the outlet port ordoor opening. U.S. Pat. No. 6,221,163, owned by the owner of the instantinvention, and International Publication No. WO2009/114798 A2, alsoowned by the owner of the instant application disclose certainstructural configurations, assembly details, materials which may beapplicable to embodiments of the inventions disclosed herein, saidpatent and publication are incorporated by reference herein

In an alternate embodiment, shown in FIG. 5 the tower configuration canbe in the form of a tower 17 made from the porous polymer material whichis friction fitted onto the base unit 7. One or more vent towers arealso preferable connected to the outlet port or ports, having astructure and function similar to that of the inlet towers, to directflow to the outlet valve for exhausting to the instrument suite.

FIGS. 3 and 4 show the interior of the tower with the porous materialfilter installed. FIG. 3 is an exploded view of the entire tower. Thisview shows the tower rear portion 9 which is a molded separate part thatis connected to the tower front portion 11 with a series of tabs 12located on the tower rear portion 9 that fit into recesses 13 on thetower front portion 11. FIG. 3 is a cut away view of the interior of thetower 6 near the base portion 7 showing the tower grommet 16, the purgegas inlet 14, and the purge gas flow path through the grommet and intothe tower flow conduit or fluid flow passageway 15 and out of the nozzleorifices 8. Purge gas enters the tower 6 through the purge gas inlet 14,passes through the gas conduit and the presence of the barrier filterconfigured as a strip of porous material 10 allows the gas pressure tobuild inside the tower until the pressure is sufficient to overcome thefilter membrane and then is dispersed through the nozzle orifices 8 intothe interior of the container at a uniform pressure and distributionfrom top to bottom. In other words, a pressure drop across the filtermaterial allows a uniform pressure along the entire length of the towerthat is greater that the pressure outside the tower in the interior ofthe container. The higher pressure along the length of the towerprovides uniform air flow out each orifice or each outlet region orequal air discharge along the length of an outlet slot. Thepressurization allows the purge gas to clean the interior volume throughlaminar flow across all of the substrates installed within thecontainer. Ideally the diameter of the tower is 0.75 inches to 2.0inches and the tower wall thickness is 1/16 inch to ⅜ inches. Ideallythe length of the tower is 70% to 100% of the interior height of thecontainer portion where the tower is located. And ideally the length ofthe tower portion is at least the distance between the lowermostsubstrate slot and the uppermost substrate slot.

The system and method of the invention are planned for use while thecontainer is waiting for the next production station or step in thefabrication process. These periods have been estimated to be about sixminutes to more than one hour long. Ideally, the container is completelypurged to desired levels of relative humidity, oxygen, or particulatesin a period of about 6 minutes or less. Relative humidity levels ofabout 0.1% or less have been achieved in about 5 minutes. Flow of thegaseous working fluid or purge gas is typically provided in thecontainer at up to 20 SCFH, and at a pressure of from about zero toabout 5 psi. Pressurized nitrogen gas and other inert gases aretypically available at pressures from about 65 to about 125 psi, and thepressure of the working gas within the system is typically controlledusing a point-of-use regulator, limiting feed pressure to the inlet ofthe container to a maximum of about 10 psi. Working pressures within thecontainer are typically about 1 psi. The gaseous working fluid or purgegas is filtered to remove particulates as small as 0.10-2.0 microns atan efficiency of about 99.999%

FIG. 5 shows the alternate embodiment of the tower configured as atubular structure formed entirely of, or substantially entirely of,porous material so that the tower portions are no longer needed. Towerportion tubular structure 17 is molded from the porous barrier materialand formed into the shape of the tower portion without the nozzle slotsin the front portion. The molded purge tower is then friction fitattached to the base portion 7. The purge gas behaves the same as in thepreferred embodiment in that the gas in held within the tower until thepressure increases to a level sufficient to permeate the barriermaterial and proceed to the interior of the container. An alternateembodiment of the molded barrier is one in which the tower in made fromthe barrier material, however sleeve 18 is added that is “C” shaped in ahorizontal cross section. This allows the purge gas to pass through thetower material and onto the substrates through the open slot running theentire length of the sleeve from top to bottom. This tower configurationallows improved directionality of the purge gas through the relativelylarge single purge slot that is adjustable by rotating the sleeve on thetower. The sleeve may provide a cap or end plug on the top of a tubularporous form that is otherwise open at the top.

FIG. 6 shows the bladder 19 which is molded out of the porous barrierpolymer material so that it fits into the tower 6 and provides the purgegas filtering and pressurization by not only covering the orificenozzles 8, but also sealing the reveal 20 where the two separatelymolded tower halves 9 and 11 are mated. The bladder shown in theillustration is molded without a back side to facilitate the moldingprocess. Therefore the bladder 19 must be inserted into the tower withthe opening away from the orifices 8 in order to work properly. Each ofthe bladders has an interior fluid flow passageway 23 that extends thelength of the bladder and the length or substantially the length of thetower. The bladders may be formed of a flexible woven polymeric porousmaterial.

FIG. 7 shows an alternative construction for bladder 19. This view,which is a cut away view of the bladder shows the different thicknessesof the walls of the bladder. The front bladder wall 22 is thinner thatthe rear bladder wall 21. This different thickness facilitates the purgegas delivery since the gas will permeate faster and easier through thethinner wall 22 which is facing the tower orifices than through thethick rear wall 21. This difference in wall thickness requires that thebladder is correctly installed. The porous material is preferably porouspolymers such as polyethylene, polypropylene, PTFE(polytetrafluoroethylene), or PFA (perfluoroalkoxy). The porosity in theporous material may be, for example, be in the form of a foamedmaterial, a fibrous material or a sintered material. The fibrousmaterial can be woven or randomly packed or intertwined. The particularshapes herein can be formed by thermal molding or, in some cases, bymachining rigid blocks or preforms of the porous material. Suitableporous polymers are available from GenPore, 1136 Morgantown Rd, ReadingPa.

Appropriate porous materials used in the purge tower to improve the flowof the purge gas of the tower into the container by by making it uniformalong the length of the tower by providing sufficient pressurization ofthe gas to build pressure within the tower prior to permeating throughthe barrier membrane filter material which provides for a uniformpressure and dispersion within the container and across all of thesubstrates. Simultaneously this material has hydrophobic characteristicssuch that it prohibits or minimizes water from entering the purge towerduring the wash cycles of the recycling procedure. Keeping water out ofthe purge tower and the purge inlet and purge outlet minimizes the timerequired for the cleaning process and keeps the container cleaner.

Referring to FIG. 8, in an embodiment, the purge tower may be formed ofa tubing portion 36 formed of porous material closable at the top end38, with a cap 40, suitably formed of nonporous polymer that can besealing fit over the top of the tubing. To direct the discharge on oneside of the tower, non porous polymer blocking sleeve 44 configured as atubing segment with a slit extending axially and formed of a non porouspolymer, my be fitted around the porous tubing portion. The bottom maybe secured to a nonporous polymer fitting 48 and connected at the bottomend to a purge port. The purge port is connectable to a purge gas sourceexterior of the container by conventional means. The purge tower may bepressurized such that flow velocities and flow volumes near the top ofthe tower are equal or substantially equal to such flow velocities andvolumes near the bottom of the tower. Ideally the diameter of the tubingportion is 0.75 inches to 2.0 inches and the tubing portion wallthickness is ⅛ inch to ⅜ inches. Ideally the wall thickness of thetubing portion is 10% to 25% of the diameter. Ideally the length of thetubing portion is 70% to 100% of the interior height of the containerportion where the tubing portion is located. And ideally the length ofthe tubing portion is at least the distance between the lowermostsubstrate slot and the uppermost substrate slot.

FIG. 9 shows the levels of relative humidity within the microenvironmentbefore and after purging with the present invention installed and withthe door removed. Measurements of relative humidity made before purgingat wafer slot #13 indicates a relative humidity level 23 of between 40%and 45%. Another measurement made prior to purging at wafer slot #1indicates a relative humidity level 24 of between 35% and 40%. Afterpurging, relative humidity measurements at wafer slot #25 indicate arelative humidity level 26 of between 20% and 25%. Another measurementmade after purging at wafer slot #13 also indicates a relative humiditylevel 25 of between 20% and 25%.

While the present invention has been described herein in conjunctionwith a preferred embodiment, a person with ordinary skill in the art,.after reading the foregoing, can effect changes, substitutions ofequivalents and other types of alterations to those set forth herein.Each embodiment described above can also have included or incorporatedtherewith such variations as disclosed in regard to any or all of theother embodiments. Thus, it is intended that protection granted byLetters Patent hereon be limited in breadth and scope only bydefinitions contained in the appended claims and any equivalentsthereof.

1. A system for transporting substrates comprising: a substratecontainer having a container portion with an opening forloading/unloading substrates, a door adapted to sealably cover theopening; a substrate carrying section disposed in the transportcontainer; an apparatus comprising an inlet tower adapted to be mountedto substrate container portion for admitting a gaseous working fluid tothe interior of the container for purging the substrate container bodywith the gaseous working fluid; the inlet tower having a length andcomprising a non porous polymer sleeve extending the length of thetower, one or more outlet regions arranged along the length of thetower, and an interior flow passageway extending the length of thetower, filter media positioned at each outlet region intermediate theone or more outlet regions and the interior flow passageway whereby thegaseous working fluid being admitted to the substrate container may befiltered as it passes through said filter media.
 2. The apparatus ifclaim 1, wherein the one or more outlet regions of said inlet towercomprises a plurality of orifices and the filter material comprises astrip of porous material that covers the orifices, the strip of porousmaterial extending in the interior of the tower.
 3. The apparatus ofclaim 1, wherein said inlet tower comprises a porous polymeric materialthat blocks water from the interior of the tower.
 4. A system fortransporting substrates comprising: a substrate transport containerhaving a container body with an opening and a wafer loading/unloadingdoor adapted to sealably cover the opening; an apparatus for admitting agaseous working fluid to the interior of the container comprising aninlet tower formed of porous barrier material surrounded by a non porouspolymer sleeve and a filter in the tower formed of porous barriermaterial.
 5. The apparatus of claim 4, wherein said sleeve of the inlettower has in cross section, a “C” shape.
 6. A system for transportingsubstrates comprising: a substrate transport container having acontainer body with an opening and a wafer loading/unloading dooradapted to sealably cover the opening; and an apparatus for admitting agaseous working fluid to the interior of the container comprising aninlet tower and a bladder portion made from the porous filter material,the bladder portion extending a length of the inlet tower.
 7. Theapparatus of claim 6, wherein said bladder has one side that is thinnerthan an opposite side.
 8. The apparatus of claim 6, wherein said bladderhas a length and is open on one side of said elongate bladder, saidopening extending at least substantially the length of the elongatebladder.
 9. A method of maintaining the environment within the wafertransport container to avoid damage to the substrates, the methodcomprising steps of: placing a porous barrier material in the purgetower apparatus covering the orifices.
 10. The method of claim 9wherein, the porous barrier material is in the form of a bladder. 11.The method of claim 9 wherein, the porous barrier material is in theform of a bladder with the back side missing.
 12. The method of claim 9wherein, the porous barrier material is in the form of the purge tower.13. The method of claim 9 wherein, the porous barrier is a strip ofporous material.
 14. The method of claim 9 wherein, the porous materialcomprising one of sintered material, fibrous material, or foamedmaterial.
 15. A method of providing equalized purging flow of a gaseousmaterial along the length of a purge tower in a substrate container, themethod comprising providing a purge tower with an interior connected toa purge gas source exterior of the container, the purge tower having aplurality of apertures, and providing porous material adjacent theapertures while leaving open a gaseous flow path the length of the towerthat is not blocked by porous material.
 16. A method of providingequalized purging flow of a gaseous fluid along the length of a purgetower in a substrate container, the method comprising providing a purgetower of a tubing portion and formed of a porous polymer, the tubingportion closed at a top end and connected at the bottom end to a purgeport; connecting the purge port to a purge gas source exterior of thecontainer, providing a pressurization of the purge tower such that flowvelocities and volumes near the top of the tower are equal orsubstantially equal to such flow velocities and volumes near the bottomof the tower.
 17. The method of claim 16 wherein the method furtherprovides adjusting the direction of the purge flow utilizing an elongatesleeve with a lengthwise slot, the sleeve conforming to and fitted tothe tubing portion.
 18. A substrate container comprising: a substratecontainer having a container portion with an opening forloading/unloading substrates, a door adapted to sealably cover theopening; the substrate container having a substrate carrying region inthe transport container and further comprising an inlet tower mounted tocontainer portion in the interior of the for admitting a gaseous workingfluid to the interior of the container for purging the wafer containerbody with the gaseous working fluid; the inlet tower having a length andone or more outlet regions arranged along the length of the tower, andan interior flow passageway extending the length of the tower, the towercomprising a filter media formed as a tubular structure extending thelength of the tower, the tubular structure having an axial fluid flowpassageway connecting to a purge port at the bottom of the containerportion for allowing purge gas to flow up the tubular structure and toexit the tubular structure along the entire length of the tubularstructure, the filter media defining the axial flow passage.